The present invention relates to a nonreturn piston slide valve wherein a slide piston forms the valve closure member. The valve comprises an outer housing of substantially rotational symmetry. Centrally in the outer housing there is arranged a streamlined inner housing also of substantially rotational symmetry. The inner housing is held in the outer housing by connecting lands thereby forming a substantially free ring space for the flowing medium. A valve seat is provided in the ring space and the valve seat is closable by a slide piston biassed by a spring and coaxially arranged in the inner housing. The spring bias is effective against the flow direction to close the valve when the flow direction is reversed or when the flow is overcome by the bias of the spring. The flow passage may be shaped in such a manner that the flowing medium is initially accelerated and then again decelerated.
In a known slide piston valve the piston is operated by means of a bellows made of an elastic material. The bellows is closed on all sides and rests with its surface against the inside surface of the hollow inner valve housing and against the piston. The bellows end opposite the end in contact with the piston is secured to the valve housing. The valve is closed by the piston in response to introducing a pressure medium into the bellows. In order to prevent the build up of a counter pressure behind the piston around the wall of the bellows, the wall of the inner valve housing is provided with one or several openings.
A valve of the just described type may be used in principle also as a nonreturn valve if the function of the bellows is omitted. The piston closes the valve under the spring bias force against the valve seat seal in response to a reduction of the flow volume or in response to a reversal in the flow direction.
It is a well known phenomenon that nonreturn valves are subject to an undesirable or troublesome flutter movement of the piston, especially when the flow passage between the piston and the valve seat is narrow or when the flow volume is small.
In order to reduce such flutter movements in a slide piston valve as described above it has been suggested heretofore to provide the inner housing with one or several apertures in the range of the maximum flow speed of the flowing medium as shown in my U.S. Pat. No. 3,993,093. According to this suggestion the flowing medium is exposed to a reduced pressure in the closed inner portion of the inner valve housing relative to the pressure of the flowing medium upstream or downstream of the inner valve housing. The reduced pressure is effective on the inside of the piston and against the pressure of the spring. Thus, it is possible to use a relatively strong or stiff spring. If now the volume of the flowing medium is reduced, the pressure inside the inner valve housing also becomes smaller thereby correspondingly reducing its effect on the piston and the counter pressure of the spring. Thus, the spring presses the piston under its own force against the seal of the valve seat in the outer valve housing. In this manner it is intended to prevent the generation of an oscillating piston movement.
Extensive tests made with the known slide piston valves have shown that the ejecting effect was rather nonuniform as a function of the piston position, especially it was not sufficiently effective when opening the valve in its initial phase. It was shown, that the pressure in the flow passage corresponded to the pressure inside the inner valve housing so that initially the pressure difference was insufficient. Further, a secondary flow could be observed in the partially opened position of the piston inside the housing behind the apertures. Such secondary flow was related to the eddys occurring in the flow passage in the area of the narrowest flow cross-sectional area between the outer jacket surface of the piston extending out of the inner valve housing and the outer surface of the inner valve housing.
The closest prior art known to applicant is applicant's own U.S. Pat. No. 3,993,093 referred to above, and the Shaw British Pat. No. 577,825. The present invention provides a significant advance over the check valve described in applicant's previous U.S. Pat. No. 3,993,093 by locating the apertures for the inner housing in the wall of the valve piston. In applicant's previous check valve the apertures were formed in stationary position through the inner housing to communicate pressure and prevent oscillating movements of the piston. According to the present invention the placement of the apertures for movement with the valve piston affords additional features and advantages including rapid valve opening followed by a hydraulic breaking or damping action as the valve piston reaches the fully open position.
The Shaw British Pat. No. 577,825 does not describe a nonreturn piston slide valve or check valve as provided by the present invention. The present invention contemplates a nonreturn valve, which is normally open in response to normal flow in a channel. When the flow rate or pressure falls below a predetermined value, the valve closes to prevent a reverse flow of the medium. Shaw discloses a pressure relief safety valve which is normally closed and which must open in response to pressure. Thus, there is a basic functional difference between the valve of claim 1 of the present application and the valve according to British Pat. No. 577,825.
Furthermore, the openings or apertures in the valve sleeve of the Shaw reference are different in structure and function from the aperture openings in the valve piston in the present invention. The openings or apertures of Shaw do not function to avoid piston vibration. Shaw provides annulus E and annulus F for such purposes. The openings in the valve sleeve of Shaw are required to permit the opening of the valve. When the valve piston A moves from the closed to the open position the volume D inside the cylinder is reduced. It is therefore necessary to provide a pressure relief aperture for outflow of fluid as a result of the reduction in volume.
On the other hand, the apertures of the present invention serve to communicate the low pressure from the annular Venturi-like flow path around the valve to the inner space of the inner housing. This beneficial communication of low pressure throughout most of the stroke or movement of the piston facilitates rapid opening of the valve. Furthermore, the apertures are advantageously positioned in the piston wall to pass into the inner housing for damping or breaking piston motion toward the end of the stroke. This avoids undesirable slamming of the valve piston during opening.
The Shaw British Patent does not include a unitary piston head and piston wall as required by the present invention. The apertures in Shaw are formed in a separate sleeve beneath the piston head. In the present invention the apertures are formed in the piston wall itself for exposure to the maximum flow zone during most of the stroke and for passing into the inner housing at the end of the stroke. Thus, the safety pressure relief valve of Shaw performs a different function from the nonreturn valve or check valve of the present invention. Because of the different structure it cannot achieve the advantages which the valve piston apertures of the present invention provide in cooperation with the inner and outer housing of the valve.